An automobile underbody member, such as a suspension member, is ordinarily fabricated in such a manner that a hot-rolled steel plate is formed into steel plate members having prescribed shapes by press forming or the like, and the members are joined by arc welding and then subjected to cationic electrodeposition coating for use.
In the case where a hot-rolled steel plate having a cationic electrodeposition coating undergoes damage on the coating due to chipping by a flying pebble, corrosion may proceed from the damage. Furthermore, on arc welding, an Fe scale is formed due to the heat input of welding on the surface of the base material in the vicinity of the weld bead toe, and the cationic electrodeposition coating may be peeled off along with the Fe scale as the underlayer due to vibration of a running automobile in some cases, in which corrosion of the base material may proceed at the peeled off position. Accordingly, it is necessary to design the strength of the automobile underbody member by taking the reduction in thickness due to corrosion into calculation. Specifically, a hot-rolled steel plate having a thickness of from 2 to 3 mm of a steel species having a tensile strength of from 340 to 440 MPa has been frequently used from the standpoint of the collision safety.
According to the demand of further enhancement of the collision safety and weight reduction in recent years, there has been an increasing need of the use of a high-strength steel plate having a strength of 590 MPa or more as a steel plate for an underbody member. Furthermore, enhancement of the rust prevention capability is also demanded for prolonging the lifetime. In recent years, moreover, good ductility and good bending workability have been also demanded for a steel plate for an underbody member from the standpoint of the formability.
PTL 1 describes a high-strength hot-dip galvannealed steel plate excellent in bending property. However, in the case where a zinc-based alloy plated steel plate is subjected to arc welding, the plated layer may be lost due to evaporation particularly in the vicinity of the weld bead toe, which is exposed to a high temperature, and an Fe scale may be formed at that portion. Thus, the problem of the ordinary hot-rolled steel plate mentioned above, i.e., peeled off of the coating along with the Fe scale is likely to occur, is not solved by the use of a zinc-based alloy plated steel plate.
A hot-dip Zn—Al—Mg based alloy plated steel plate has been known as a plated steel plate that has higher corrosion resistance than an ordinary hot-dip galvanized steel plate, and has been applied to various purposes. By using a hot-dip Zn—Al—Mg based alloy plated steel plate as an underbody member, the functions inherent to the Zn—Al—Mg based alloy plating, for example, a film with high protecting capability derived from the plating components is easily formed even in the case where the coating is peeled off along with the Fe scale formed on arc welding, may be exhibited and thus the corrosion resistance in the vicinity of the weld bead toe maybe largely improved in comparison to a member using an ordinary hot-dip galvanized steel plate. However, in the case where a hot-dip Zn—Al—Mg based alloy plated steel plate is welded, there is a problem that liquid metal embrittlement cracking is liable to occur in comparison to an ordinary galvanized steel plate. The liquid metal embrittlement cracking is such a phenomenon that the plated metal in a molten state on the surface of the base material immediately after arc welding penetrates into the crystal grain boundary of the base material, which is applied with a tensile stress, so as to cause cracking of the base material on cooling.
PTL 2 describes a Zn—Al—Mg based alloy plated steel plate improved in resistance to liquid metal embrittlement cracking. However, the plated steel plate described in PTL 2 may not be necessarily satisfactory in bending workability. As a result of investigations made by the present inventors, it is considered that the plated steel plate described in PTL 2 has, as the metal structure of the steel base material, a two-phase structure of ferrite as the main phase and martensite, and improvement of the bending workability may be made difficult since the crystal grain diameters of ferrite and martensite are not sufficiently small.